A garbage collection process for collecting unused data areas and reserving the unused areas as consecutive data areas has been developed. For example, as a related art, image data is divided into data segments of the same size, which is smaller than or equal to the size of each of a plurality of partitioned storage areas of a hard disk, and is compressed. As another related art, the following technique has been developed: a candidate of a file to be compressed is presented to the user based on the file management information and the user setting, a specified file is selected as a file to be compressed and is compressed, and the original file is overwritten by the compressed file.
Japanese Laid-open Patent Publication No. 9-123539 and Japanese Laid-open Patent Publication No. 2001-43115 discuss the above-described technologies.